This invention relates to visual aids for the visually handicapped and, more particularly, to a prismatic lens system which provides users suffering from diplopia with a binocular vision field.
All of the motions of the eye are accomplished by six muscles, called the extraocular muscles because they are located outside of the eye. There are four rectus muscles, one attached to each side and one to the top and bottom of the eyeball, about a half centimeter from the cornea. Depending on their location, these act to rotate the eyeball upward, downward, right, and left. Each eye also has a pair of oblique muscles which are attached to the eyeball behind the outside rectus muscles (i.e. behind the right rectus in the case of the right eye and behind the left rectus in the case of the left eye). When the oblique muscles pull against one another, they act to rotate the eye like the steering wheel of a car. The visual axis of each eye is an imaginary line extending from a viewed object to the retina and passing through the air, corrective lens (if any), cornea, aqueous fluid, pupil, crystalline lens, and vitreous fluid.
The binocular image field obtains from an intersection of the right and left conical spaces emanating from each eye. As shown in FIG. 1, when both eyes are aimed directly forward, the coincident centers of the visual space for each eye comprise a simplified binocular field 2. The total horizontal and vertical fields are greater than the binocular field as shown in the shaded area 4. Occlusion of part of the visual field is caused by facial features such as the nose, eyebrows, and cheeks.
Both eyes are nearly in constant motion, thereby enlarging the field of view and eliminating any gaps. The eyes cannot be independently moved but are synchronously deflected in what is known as "saccadic" movement. When two eyes fixate an object, they are brought to simultaneous focus with the separate visual axes directed through the same point in a process called "convergence". The average time to change convergence is about 165 msec. The nearest point of convergence is on the order of 75 mm from the eyes, with anything object closer being viewed as a double image.
To appreciate the full value of depth perception, very precise and intricate alignment of the eyes is necessary. Horizontally, the alignment must be within a few degrees; vertically, even less. If the six muscles controlling each eye cannot point the visual axis of each eye at the same spot within this range, the stereoscopic effect will be diminished or lost. The images may, in fact, be so diverse that the brain cannot fuse them into one image and the person sees two images, a condition called diplopia.
In adults, eyes can become uncoordinated and diplopia can result after muscle-weakening diseases, including myasthenia gravis, multiple sclerosis, stroke. Additionally, diplopia an result from a brain disorder such as a brain tumor, from a cataract, or from mucus caused by an infection. Occasionally, an interval of double vision is experienced by some patients following eye or brain surgery.
If both eyes see well and require little lens correction, a person with a disease-weakened eye muscle may see two images of the world for the rest of his or her life. If an unaffected eye also happens to see better than a weakened-muscle eye, the better eye may be relied on more and more and become so dominant that the weakened eye actually starts deteriorating as a result of the inhibition imposed on it by the brain.
Visual aids utilizing prisms are known. Currently, persons suffering from strabismus, a condition in which each eye points in a different direction, are prescribed glasses utilizing prismatic lenses in which the base is directed upward, downward, outward, or inward. These prisms are intended to strengthen the affected muscles of the deviating eye and thereby cause the eye to straighten over time. The unit of prismatic deviation of the prismatic lenses used in such glasses is the prism diopter. A one diopter prism is one which deflects a beam of light a distance of one centimeter on a plane placed normal to the initial direction of the beam and one meter away from the prism.
Prismatic glasses can be very effective in strengthening the eye muscles of otherwise healthy patients and can thus be employed to straighten the eyes of individuals, particularly young children, suffering from conditions such as strabismus and amblyopia. However, as indicated above, diplopia is often associated with degenerative diseases in which affected muscles grow progressively weaker and the deviation gets larger. In such cases, the prismatic visual aids described above are ineffective.
Accordingly, it is an object of the present invention to provide an adjustable prismatic lens system for a visual aid apparatus which enables a patient suffering from diplopia to retain and maintain binocular vision despite progressive changes in the muscular coordination of the eyes.